Q: What does SPF mean?
A: SPF stands for Sun Protection Factor. SPF tells you how long you can stay in the sun without getting burned while wearing that sunscreen, compared with how long you can stay in the sun before you burn without the sunscreen.
For example, if it typically takes you 15 minutes to burn without sunscreen and you apply an SPF 10, it will take you 10 times longer to burn, or 2.5 hours. A higher SPF doesn't mean better sun protection. It just means that you will be protected longer. But that is only valid up to a certain extent. You have to apply enough of the sunscreen, and other factors, such as sweating or water, can decrease its effectiveness.
To be safe, no matter what SPF you use, it's best to reapply at least every two hours, as well as after swimming or sweating. The American Academy of Dermatology recommends daily application of SPF 30.
SPF only measures protection against the ultraviolet B rays (UVB). UVA rays are also dangerous, so you need a sunscreen that is broad spectrum so it covers both UVA and UVB.
— Dr. Cara Hennings, UT Erlanger Dermatology; member, Chattanooga-Hamilton County Medical Society